Around the early 1900's a new, growing use of electrical and communication wiring was seen throughout the nation. Open office spaces with many workers lead to the development of a system of ducts or raceways embedded in the floor structure to distribute the wiring. These systems were called underfloor raceway systems, or underfloor ducts.
In 1912, H. S. Walker and H. N Walker opened Walker Brothers Co. in Philadelphia, Pa. By the early 1920's Walker Brothers introduced the first all steel underfloor raceway made from a rectangular metal tube about 1 inch high and 2 inch wide, called Walker #1 duct.
Along the length and at the top of each raceway round hollow hubs about ¾″ diameter could be installed, usually 24″ on center. The height of the hubs was about ⅛″ less than the concrete toping over the duct. The hub was fitted with a recessed cap to prevent entrance of concrete. When access to the raceway was desired, concrete was chipped out and the cap was removed.
Wires were then pulled through the raceway, up through the hub and into an outlet box or floor fitting, sometimes called a pedestal or doghouse.
In addition to Walker #1 duct, Walker introduced a larger duct, 1¼″ high and 3⅛″ wide, commonly known as Walker #2 duct. A combination of #1 and #2 ducts is shown in U.S. Pat. No. 2,063,569. Outlet hubs more commonly called “preset inserts” with a 2″ internal diameter conduit thread could now be installed on Walker #2 duct. As larger communication cabling became popular, #4 duct 1½″ high by 6½″ wide, was introduced in the 1950's. U.S. Pat. No. 3,061,663 shows the versatility of a duct system using larger sizes of duct. Walker #1 duct was discontinued in the 1950's, but Walker #2 and #4 duct in ten-foot lengths with or without factory installed 2″ round preset inserts continues to be sold today.
Another old underfloor duct system specifically made for renovation projects is called a “flushduct” system. The system includes one, two, or three #2 ducts welded together and installed flush with the top surface of the concrete floor. An existing floor is cut to create a channel for this system. The Walker junction boxes for this system are still made from cast iron and only pedestal type outlets can be installed on flush duct. Cutting the floor of an older building is easier because of thick floors. Cutting the floor of newer buildings may weaken the floor slab so this system has limited usage.
Typical in-floor systems include parallel and perpendicular runs of duct connected at their intersections by junction boxes. Until the 1950's most junction boxes were made of cast iron. U.S. Pat. No. 2,919,827 describes a Walker junction box with a sheet metal top and a cast iron base.
The installation of a typical system begins with locating junction boxes in a row, usually 4 to 6 foot on center and installing duct between these boxes. These rows of duct and boxes are commonly called “feeder ducts” and usually begin at the electrical or communication rooms. Junction boxes are marked with arrows to indicate the feeder duct direction. Perpendicular to these rows, “distribution ducts” are installed into the junction boxes and provide a cable pathway to the workstations via the outlet hub and floor fitting. One way to visualize an underfloor layout is to think of the feeder duct runs as “highways” and the distribution runs as “side streets.”
Conventional “single level” junction boxes are square and have equally sized duct openings at all four sides. U.S. Pat. No. 1,925,849 provided greater wiring capacity in the junction box and made a distinction between feeder and distribution (branch circuits) wiring. However, this all-cast iron box was expensive to produce and required a floor slab thickness about twice as thick as most single level boxes.
The need for more feeder duct runs than distribution runs in a single level junction box can be seen in the following example.
Assume a distribution duct run of No. 2 Walker duct is sixty foot long. If workstations are installed six feet on center and each of the 10 workstations require two duplexes each, 20 duplex receptacles would be installed along the 60 ft. run. If four receptacles are installed on each circuit, 5 circuits or 15 conductors would be required (assume two current carrying conductors and one ground to each circuit). If the conductors used are 10 AWG, THHN the wires only fill the duct to 10% capacity (a walker #2 duct can contain 63 10 AWG conductors at 40% fill).
Because forty percent fill is the maximum allowed by the National Electrical Code, before ampacity and derating adjustments, it is apparent that just four runs of distribution duct would fill one No. 2 Walker duct feeder duct to forty percent. However, a raceway filled to forty percent does not leave space for changes and makes wire pulling very difficult. Industry practice is to fill the raceways to twenty percent fill. Therefore only two distribution runs in this example would fill one feeder duct. Most systems solve this problem by adding more rows of boxes.
Adding conduit feeds to the corners of boxes is another conventional way to boost feeder capacity for two duct single level systems. However, routing wires to the interior compartment of a box made for three or more ducts reduces the capacity of the two outboard duct runs, unless the depth of the box is increased. Conduit feeds can also be installed into the unused openings in the box or at the ends of duct runs. In shallow floor slabs, conduit feeds may not be possible if conduits must be routed below the duct system. Conduit feeds will also encounter a number of turns on the way to the electrical room making pulling of wires more difficult. Three U.S. Pat. Nos. 2,063,569; 2,919,827; and 3,013,690 have corner conduit provisions.
Another way to increase feeder duct capacity is to use a junction box with #4 ducts as the feeder duct and reduce the size of the distribution duct opening to fit a #2 duct. However, according to changes in the 2005 National Electrical Code regarding the derating of conductors using more than 40 current carrying conductors in a raceway such as a #4 duct would require larger and more expensive wires, which is not a preferred option.
Prior to 2005, the code did not specifically state that conductors in underfloor ducts should be derated. The installation and function of an underfloor duct system is quite different from round conduits and other raceways. In addition, testing by Underwriters' Laboratories, Inc of conductors in conduits vs. underfloor ducts and cellular raceways showed that higher currents could be carried by the wires in underfloor ducts. Nonetheless the same derating factors now apply. Consequently, the code permits only seven 12 AWG conductors in a raceway for 20 AMP circuits, and a maximum of only 40 10 AWG conductors for 20 amp circuits.
While 32 conductors fill the Walker #2 duct to 20% (the industry standard), as previously discussed in the example, only two runs of distribution ducts will fill one feeder duct run.
Two level underfloor duct systems, also called pyramid systems have larger, multiple feeder ducts on the lower level and were first designed to contain large quantities of phone cables.
In the 1960's, 25 to 100 pair phone cables ⅜″ to ⅞″ diameter were essential to provide telephone service to large open office spaces. Conventional single level systems had difficulties containing these large cables, but two level boxes provided the space and quantity of #4 feeder duct runs as required to contain these cables. However, increasing the floor slab depth to accommodate the system could increase cost and increase the height of floors above grade level. U.S. Pat. No. 3,264,791 attempted to address this problem. This patent also describes the problem of single level duct systems in paragraphs four and five, beginning with the third sentence in paragraph four. U.S. Pat. No. 3,428,203 and U.S. Pat. No. 3,784,042 also pertain to two level underfloor duct systems.
Cellular Metal Floor Raceways invented in the late 1940's by the HH Robertson Co. is another type of two level Infloor system used mainly above grade for steel frame buildings. The hollow spaces created by the corrugated sheet metal decking are used as distribution cells for wire and cable. The decking also serves as a platform for the concrete floor. A #4 duct, called a header duct was first used as the feeder duct raceway. It is installed on top of and perpendicular to the metal decking.
Because office areas and needs for more wiring to each workstation were increasing, multiple runs of #4 duct were required. Round hubs, about 6″ in diameter called hand holes extend 1″ or more above the 1½″ high header duct and have removable access covers. The hand holes were positioned on the header duct over predetermined cells of the floor decking and 2″ to 4″ diameter holes were drilled in the floor cell for transition of wiring from the header into the floor cells. The hand hole function is similar to a two level duct underfloor duct junction box. Concrete depths of 2½″ or more is poured on top of the decking and finished flush with the top of the hand holes. When used with standard three inch deep cellular decking the total floor slab will be 5½″.
Holes for floor outlet fittings are drilled through the concrete and into the top of a cellular floor raceway after a tenant has determined the final location of workstations. More recently, dual and triple service outlet boxes are installed on the metal decking prior to the concrete pour on a five-foot grid similar to preset inserts on underfloor duct.
As wiring needs continued to expand in the 1960's, a “trench header” as wide as 36″ began replacing multiple runs of #4 ducts as the feeder duct for a cellular floor system. Rather that a hand hole cover over a predetermined cell, a trench header has removable steel cover plates, ¼″ thick and 24″ long installed flush with the finished floor for its entire length.
Runs of trench duct can be 150 feet or more. Holes are field drilled into the bottom of the trench duct and the top of the metal decking for routing of cables to the workstations. Field cutting of holes was eliminated by omitting the trench header bottom plate and factory punching holes in the cellular metal floor panels as shown in U.S. Pat. No. 3,721,051. A trench header, typically 2½″ high can contain large quantities of cables and wires. The wiring can be “laid in” by removing all covers prior to erecting walls and work stations.
With its advantage for communication cables, trench header also became popular for use with underfloor ducts. The trench duct is generally used as the feeder for the underfloor ducts. Underfloor distribution ducts are either installed below the trench as a two level system or into the side of a trench as a single level system. Because of larger compartments and greater quantities of electrical conductors, larger wires or smaller compartments are usually required to meet new code requirements. This did not exist prior to 2005. Some architects do not specify trench duct because cover plates can rattle and have a hollow sound when they are walked on.
Cellular metal floor panels can also be used as a single level duct system using the panels as both feeder and distribution raceways. A sheet metal frame at the intersections creates the junction box. This design is described in U.S. Pat. No. 3,453,791.
Walker offers three different sizes of three compartment cellular underfloor systems and junction boxes. The product is called WALKERCELL® and is used on grade or two pour concrete slabs. The smaller power cell in the center and an easy method to add power feeds to the junction box is not offered.
Although a number of new Infloor system products and features have been introduced, the single level junction box has seen few changes in the last ninety years. Until several years ago the most common junction box sold was about 2½″ high with duct positioned one inch below the finished floor. The underfloor duct was furnished with ⅞″ high, two inch round hubs for use with pedestal type outlets.
The tunnels inside this box separate the services and are divided horizontally to provide equal sized compartments for feeder and distribution ducts. Because of the boxes shallow internal height, a smooth, straight pathway through the box is not provided in either direction. Examples of this are shown in five of the referenced patents of this invention. Regardless of the height of the box the size and shape of the tunnel dividers does not change with conventional single level boxes.
Rather than 1″ of concrete over the duct, today's systems use 1½″ or more. Flush outlet fittings have replaced the pedestal type and this product needs 1½″ or more concrete for best function. A Walker flush outlet fitting is described in U.S. Pat. No. 6,072,121. The floor covers for these outlets are the same as Walker's concrete floor boxes and can be seen in most shopping malls and retail stores.
1½″ concrete depth is much less likely to crack than 1″ deep. Stress cracks and fractures are a common problem with underfloor ducts, and very unsightly for projects that have bare concrete floor such as some of the big box retailers. Some applications specify six inches of concrete over the duct to prevent cracking.
Another shortcoming with existing boxes is strength and stability. A junction box and duct must be securely anchored and maintain it preset height during concrete pour. The weight of wet concrete can easily displace or sink a system that is not properly installed. This is a common problem because current boxes use long machine screws with sheet metal feet to support the box and maintain its position during the concrete pour. Some floor slabs may be 12″ thick or more with 6″ or more of concrete poured over the duct.
Since junction boxes are in the concrete floor before walls are erected, the access covers must be able to support construction loads. After building construction is complete, furniture, office equipment, supplies, and other materials could be moved over the access covers. Textile mills, aircraft maintenance facilities and casinos are examples of applications that need even thicker access covers. Steel covers ½″ thick or more have been used.
Ideally, the top of the junction box will finish flush with the finished floor. However, concrete pours are not always perfect around the box and if the box shifts or sinks it must be fixed. For these reasons access covers may be configured to be adjustable after the concrete pour.
The leveling and support method for current junction box access covers involve four machine screws installed on ledges or into internally threaded fasteners located in the four corners of the box. For additional support, adjustable posts fastened to the bottom of the box may be used.
Loads placed on the cover plate are transferred from the screws to the ledges or base of a conventional box, and if the bottom of the box is not resting on a solid surface the loads will deflect and possibly damage the cover plate or other internal components. To allow for variations in floor slab thickness, the box may be at least slightly less in height than the floor slab.
For example, one type of shallow floor construction involves building a structural floor slab, installing the duct system on top of this slab, then covering the system with concrete. This first slab is not precisely leveled and has a rough finish. When the box is adjusted up to finished floor elevation, a slight air space could be present and allow damage or deflection to the junction box. Often the installer is not aware of how the internal post supports the access cover and does little or nothing to fill the gap below the box. If the space below the box is 1½″ or more concrete can be worked under the box. However, if this step is improperly done some of the box, especially the center, may not be supported. There are other examples of floor constructions, such as slab on grade, where it is difficult to provide a solid surface under the junction box.
Placing the box down on a solid surface and adjusting the access cover before the pour is generally not an option because the duct would be further down in the floor making access to the outlet hubs more difficult. The four adjusting screws only adjust the cover up ½″, then they must be replaced with longer screws. Also, if the box has more than two #2 ducts or one #4 duct it is provided with five additional internal adjustable posts and the cover plate must be removed to adjust these posts. If not adjusted, the chances for damage during construction are increased.
In addition, the frame of the box and the edge of the cover may be at different elevations resulting in concrete cracks. Finally, this entire adjustment procedure may need to be repeated if the concrete pour is not level with the top of the box.
As previously mentioned, conventional single level junction boxes are square with an equal number of openings at each side. Walker offers 12 different duct combinations: single #2 duct up to five #2 ducts; a single, double and triple #4 ducts; one #2 with one #4; one #2 with two #4's; two #2's with one #4; and a box with one #4, #2 and #4. Eight heights from 2½″ to 6″ in ½″ increments are standard.
Walker offers these boxes in four different styles, resulting in 384 offerings. In addition custom heights have been made up to 24″ deep and other special features are possible. Each change requiring a different combination of ducts requires a complicated change to the internal tunneling and other components. In one such example, there are 27 parts, not including hardware, required just to produce the box enclosure and tunneling in a Walker two duct box. As discussed, feeder ducts and junction boxes are installed in rows, typically four to six foot on center. Ducts fit between the boxes and are field cut from standard ten-foot pieces, or on larger projects can be factory cut to the exact length. For the past ninety years, underfloor duct boxes have had recessed openings for the insertion of ducts as shown in all cited patents. While this is fine for a distribution run, it can be difficult when installing feeder ducts from one box to the next. After installing ducts into the first box, the next box and the duct must be jockeyed into position. This can be difficult because a box such as one that has two #2 ducts and one #4 duct weighs 56 lbs.
Underfloor junction boxes provide the means to transition between distribution and feeder ducts. Wires are pulled from the panel to the box and from the box to the outlet hub using a fish tape or pull string. This procedure can also be done in reverse. In either case, the junction box is the pivot point for the wire and cable installation. Of course, this requires the removal of the access cover on the box. This can be a difficult task depending on the type of floor covering installed on the junction box. Junction box covers are fitted with floor finish trim frames and may have the floor finish attached to the access cover. This can make cover removal extremely difficult. A lawsuit was filed by a telephone worker who was injured while trying to remove a junction box cover in a shopping mall that had 1″ marble attached.
Another problem that exists today concerns the entrance of water on the floor into the junction box. With most projects, after the floor slab is poured there is no protection from the elements. Water that can get into the junction box and duct system and do serious damage to the galvanized steel. The water may combine with the chemicals in the concrete and the lack of air circulation may damage all of the galvanized steel in a short time. To address this concern, duct tape may be installed along the side and over the top edge of the box to keep out the water on all Walker junction boxes. Although the duct taped junction box may work when properly installed, the tape must be removed during maintenance, and this can be difficult while leaving a residue or removing the paint from the access cover. A good concrete finish is also inhibited and the tape installed on the side of the box remains in the concrete and leaves a ragged edge.
A patent dealing with underfloor junction boxes, U.S. Pat. No. 4,931,597 describes a junction box using removable tunnels, extension collars and random placement of duct separators. The described arrangement reduced the number of junction box permutations and provided a horizontally and vertically divided tunnel insert with equally sized passageways through the box. Four or five smaller ducts could be provided on the feeder duct sizes, but the same quantity of openings occur on the distribution sides. This wastes material and requires closing off several openings if only two distribution duct runs are required. Many parts and pieces are required to produce this box. This system was introduced by the Square D Company in 1988. Square D discontinued this product offering in 2005.